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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

                            
000
FXUS65 KBOU 222059
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
259 PM MDT Thu Aug 22 2019

.SHORT TERM...(This evening through Friday)
Issued at 250 PM MDT Thu Aug 22 2019

Afternoon convection underway across mountains and adjacent 
plains. Strongest activity now across Douglas and Elbert counties 
with storms firing on boundary and will be moving into richer and 
moist airmass. With initial weak convection already has moved 
through Denver area, expect any additional activity will be weak 
and mainly gusty winds as low level moisture has mixed out as pw 
values have dropped under a half of an inch and dewpoints are down
into the 30s/lower 40s. Strongest storms still being targeted 
over the northeast plains, especially along the CYS ridge and 
then propagating east and southeast over the Colorado northeast 
plains. Dewpoints holding in the upper 50s/60s over the far plains
with decent southeast winds on the plains. ML CAPE still 
projected upwards of 2500j/kg. Despite the weaker mid level winds 
of 10-20kt, low level flow should suffice in a few strong to 
severe storms over the plains through early evening. 

For Friday, one more day of a severe weather threat on the plains 
before drying comes in over the weekend. There will be a decent 
upper trof which will be moving across the northern rockies with the 
tail end moving across northern Colorado on Friday with weak upward 
ascent through the period. Like today, the best threat of severe 
storms will be north and east of Denver where the richer low level 
moisture will reside. Appears low level moisture will mix out over 
the Front Range with the main threat of gusty winds. SPC has again 
targeted the northeast plains for a marginal threat of storms for
damaging winds and hail.

.LONG TERM...(Friday night through Thursday)
Issued at 250 PM MDT Thu Aug 22 2019

Dry northwest flow aloft looks to be the rule across northern 
Colorado in the extended period. A series of cool surface surges 
will keep temps closer to seasonal average for much of next week 
with bouts of heat and cool. Critical fire weather conditions may 
develop for the Sunday, Monday and Tuesday timeframe.

Any semblance of organization to the defined subtropical ridge 
over the southern US is mostly gone by Friday evening as an 
enormous stable ridge of high pressure continues to develop over 
the eastern Pacific. This eventually makes for a more progressive 
flow pattern across the northern US as the eastern edge of this 
ridge begins to influence the western and southwestern US for the 
weekend and into next week. The first feature to affect the 
region will be a passing short wave trough riding along the 
US/Canadian border. This will send a cool front southward onto 
the NE CO plains late Friday evening. Showers and storms which 
initiate Friday afternoon should interact with this cool boundary 
on the far eastern plains Friday evening, so scattered storms 
should keep going late into Friday evening and early Saturday as 
they make their way out of state to the east.

The exiting of this trough early Saturday will usher in the 
beginning of an extended period of dry northwest flow across the 
central Rockies, including NE CO. For Saturday, will expect 
cooler temps and drier conditions, especially over the mountains, 
foothills and I-25 Corridor. Moist southeasterly surface return 
flow, on the west edge of the central US low level jet, should 
supply enough moisture for a few strong storms on the far eastern 
plains Saturday afternoon and evening. Despite the drier air in 
the low to mid-levels over the region Saturday, will not expect 
any critical fire weather concerns at this time, mainly for lack 
of widespread strong winds over the higher terrain.

A tightening of the mid-level height gradients develops for Sunday 
into Sun night as another weak trough passes across the northern 
Rockies and the high plains region of MT and the Dakotas. This 
will increase the northwesterly winds speeds in the 700-500 layer 
for Sunday. Forecast soundings indicate 20-30 kts of northwest 
flow near 700mb, so with low RHs over the high country, critical 
fire weather conditions may be present for the high terrain Sunday
afternoon and potentially out onto the plains.

As the short wave trough moves east into the Minnesota-Manitoba 
region, this sends down a cold front into NE CO for Monday. This 
is mostly just a cooler temp winds shift as limited moisture and 
low level stability will be associated with it. This should make 
Monday the coolest day of the week with temps only reaching into 
the 70s, which would be 10-15 degF below seasonal average. With 
moderate northwest flow aloft persisting over the state, critical 
fire wx conditions will likely exist, primarily over the mountains
and high parks. Cooler temps and higher RHs for much of the day 
should keep the plains out of wild fire conditions.

Temperatures will rebound Tuesday as the cooler air from Monday 
is mixed out from stronger surface northwesterly flow as a weak 
lee trof briefly develops by afternoon helping this NW to SE low 
level gradient. Will expect another dry day Tuesday, and with 
strong winds over the higher terrain, will need to monitor if 
critical fire wx conditions spread onto the lower elevations 
ahead of another surface cool front late Tue into early Wed. At 
this time, this looks to be a rather strong northerly push of 
cooler air. This sharper surge/wind shift contains limited 
moisture, but should take temps back to near or below seasonal 
average for Wed into Thur.

What happens next, Wednesday and beyond, will be influenced by 
the northwesterly track of Tropical Storm IVO in the EPac. This 
is projected to move well west of the California coast, but may 
allow the subtropical high to re-establish again over Mexico and 
the southwestern US for late Tue thru Thur. Northwest flow aloft 
will still prevail over the central Rockies for the Wed-Thur 
timeframe, but the building ridge will create weaker flow aloft 
allowing for mid and upper level moisture to draw northward into 
mid week for increased PoPs especially over higher terrain.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 226 PM MDT Thu Aug 22 2019

Stronger convection has already pushed east of the terminals which
has already mixed out the higher low level moisture. Any remaining
threat will be gusty winds with weakening showers coming off the
foothills. So have removed the TS from the local terminals but
left in a tempo for gusty winds through 00Z. No impacts expected
from later this evening and overnight.  

&&

.FIRE WEATHER...
Issued at 226 PM MDT Thu Aug 22 2019

Critical fire weather conditions still look possible for the 
period of Sunday through Tuesday. The main areas set for elevated
fire conditions would be the mountains, high parks and eastern 
foothills each day. A cool front will push onto the plains early 
Monday which will allow for cooler temperatures and higher 
relative humidity across the plains and lower foothill areas. For 
Tuesday, much warmer temperatures are expected across the lower 
elevations along with the potential for northwest winds near 25mph
near the foothills and Cheyenne Ridge.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Entrekin
LONG TERM...Fredin
AVIATION...Entrekin
FIRE WEATHER...Fredin